Polycrystalline copper indium gallium selenide (CuIn(1-x)Ga(x)Se2 or CIGS) based direct bandgap semiconductors are strong candidates for low cost, high-throughput solar energy harvesting thin film photovoltaic devices. State of the art CIGS devices (X=0.3), as shown in FIG. 1A, demonstrate 15-18% power conversion efficiencies with best cell performance reaching 20.4%. (Repins, M. A. et al. Prog. Photovoltaics Res. Appl. 16 (2008) 235, “EMPA announces 20.4% efficient thin film CIGS-on-polymer cell.” PV Magazine.com, January 2013; incorporated by reference in its entirety). However, this is well below the (32.8%) Shockley-Queisser theoretical maximum predicted for the identical (X=0.3) material composition with 1.15 eV bandgap (S. Siebentritt, Solar Energy Materials & Solar Cells 95, 1471-1476 (2011)). The main reasons for this disparity are due to the effects of parasitic layer absorption and charge recombination at the interface and the bulk of the CIGS layer.
With regard to the parasitic layer absorption, apart from the light absorbing active layer of CuIn(1-x)Ga(x)Se2 in CIGS cells, there are several other non-active layers for charge-collection, electric-field formation and passivation. As shown in FIG. 1B, conventional electron extraction and window/passivation layers in CIGS PVs are fabricated using doped degenerated-oxide layers, such as aluminum-doped ZnO (AZO), Indium Tin Oxide (ITO) (˜160 nm) and intrinsic-ZnO (iZnO) (˜80 nm). Chemical bath deposited (CBD) CdS (˜50 nm) is universally used as the conventional n-doped layer in CIGS, which form the built-in field for charge extraction with the p-doped CIGS layer. Even though these layers have high optical transmission within the longer wavelengths (up to 1100 nm), there is a significant drop in transmission in the shorter wavelengths (below 500 nm for AZO, below 400 nm for iZnO and below 700 nm for CdS, respectively). These incremental reductions in transmission cumulatively reduce the amount of light transmitted to the CIGS active layer. As a result, a significant drop in the external quantum efficiency is seen in the state-of-art CIGS cells in the lower wavelengths, limiting the overall photocurrent produced by these devices. Overall the cumulative optical loss in the parasitic layers leads to nearly 5% absolute loss in the power conversion efficiency.
With regard to the charge recombination effects, the composition within the CIGS active layer is highly inhomogeneous in the directions perpendicular to the substrate, as well as in the in-plane direction. Some degree of inhomogeneity, for example the graded composition of Ga ratio and Cu-poor top-layer in CIGS, is considered beneficial for the performance (i.e. improving charge collection efficiency, reducing defects) of the CIGS. However, exact replication of these favorable conditions is not always possible and the resulting inhomogeneity leads to an unfavorable disorder in the CIGS active layer, especially in large area CIGS solar cells. Such disorder leads to the formation of defects enhancing CdS/CIGS interfacial and bulk charge recombination within the space-charge region of the device. These charge recombination effects significantly lower the open circuit voltage (Voc), Fill Factor and the short circuit current density (Jsc) in CIGS cells and also create non-uniformities in performance of CIGS based modules, which increases their cost.
A solution to overcome the above-described inadequacies and shortcomings in the present CIGS solar cells is desired. In particular, it would be desirable to produce a CIGS solar cell device with reduced parasitic layer absorption and reduced charge recombination effects.